docs: def infer_x(self, y):
          """Infer probable x from input y
  
          @param y  the desired output for infered x.
          """
          assert len(y) == self.fmodel.dim_y, "Wrong dimension for y. Expected %i, got %i" % (self.fmodel.dim_y, len(y))
          self.goal = np.array(y)